The invention relates to molded case circuit breakers of the type employing a movable and a fixed contact and an operating mechanism arranged to open and close the contacts under the influence of an operating handle and a trip mechanism. The trip mechanism and operating mechanism are similar to that described within U.S. Pat. No. 3,605,052 to Herbert M. Dimond et al. and to U.S. patent application Ser. No. 479,616 entitled "Electric Circuit Breakers Having Fast Short Circuit Response". Which patent and application are incorporated herein for purposes of reference and which contains a detailed desciption of the operation of both the trip and operating mechanisms.
The contact assembly within the Dimond et al. patent includes a contact spring for providing a high contact force between the contacts to ensure a minimum resistance to the transfer of electrical current therebetween. As described within the aforementioned patent, the contact arm is mechanically linked to a crossbar which in turn is tripped in a counterclockwise direction to separate the contacts. Under severe short circuit test conditions the contacts are held in a closed condition under the influence of the contact spring until the trip mechanism is able to rotate the crossbar and separate the contacts. Until the contacts become separated, the short circuit current let-through transfers through the various current carrying elements of the circuit breaker. The thermal and mechanical effects created by the high let through energy could conceivably cause damage to the elements if these energy values are excessive.
The purpose of the instant invention is to provide a contact arm assembly which rapidly separates the contacts upon short circuit current conditions to substantially decrease the thermal and mechanical stresses on the circuit breaker elements. This permits a higher safety margin for each circuit breaker element, and in some cases, some elements may be designed to be more cost effective and still maintain a high safety margin.